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The conversion of prothrombin to thrombin during normal coagulation proceeds with ever increasing velocity until at the moment of clotting, or shortly thereafter, thrombin evolution is explosive. This phenomenon, referred to as the "autocatalytic" nature of thrombin formation, remains unexplained. It is evident that abnormalities in this reaction may underlie the pathogenesis of certain hemorrhagic disorders on the one hand, or of excessive intravascular coagulation, on the other. Considerable progress has recently been made toward elucidating the mechanism of the autocatalytic process by the discovery of substances which, arising during coagulation, accelerate, activate or otherwise act as ancillary agents in the conversion of prothrombin by thromboplastin plus calcium. Ware and his colleagues (1-4) have separated a protein fraction from bovine serum, serum Ac-globulin, which is said to arise from the interaction of thrombin with a relatively inert precursor, plasma Ac-globulin. This moiety increases the velocity of thrombin formation as well as the amount of thrombin formed, in an isolated system containing prothrombin, thromboplastin and calcium. Owren (5) described the evolution during clotting of a substance with similar properties, Factor VI, and Alexander and his associates reported on a prothrombin conversion accelerator in serum, spca (6-9). Milstone (10), too, observed the transformation in the first phase of clotting of a plasma globulin, prothrombokinase, into an active enzyme, thrombokinase, which accelerates not only prothrombin conversion but also its own elaboration. A "prothrombin-converting factor" in serum has also been described by Jacox ( 11 ).

THE PROTHROMBIN CONVERSION ACCELERATOR OF SERUM (SPCA): ITS PARTIAL PURIFICATION AND ITS PROPERTIES COMPARED WITH SERUM AC-GLOBULIN 1